The need for robust, high temperature, in-cylinder pressure measurement has long been a desired feature for the optimization of combustion engines. This is due primarily to the close correlation between instantaneous cylinder pressure and the efficiency of the combustion process. Combustion pressure, when detected in all cylinders, can provide the most deterministic information about engine performance. In order to implement the above strategies as well as fulfill future emissions and energy consumption requirements for vehicles, new types of combustion sensors will have to be developed.
Silicon micromachined sensor devices cannot withstand the harsh high temperature and chemically aggressive environments in modern engines (100° C. typical). Piezoelectric-quartz pressure transducers have been used for decades in engine development and calibration. However, they are generally not suited for implementation in production engines as they are susceptible to electromagnetic interference (EMI), tend to have a limited lifetime and are expensive. Lower cost piezoceramic devices, such as spark plug washers, do not typically offer the high accuracy under all engine operating conditions, are also subject to EMI interference and tend to have durability problems related to alloy separation, selective oxidation and diffusion when used in production engines. Other techniques, such as ionization probes, which measure flame propagation, usually do not provide as much useful information on the combustion process as do cylinder pressure measurements.
In contrast to electronic-based pressure sensing, fiber optic sensors are well suited to applications characterized by high temperatures and the high levels of EMI encountered in engines. Current techniques for using fiber optic sensors in engines employ either the microbend concept or use a Fabry-Perot modulator. The microbend concept generally requires extremely high pressures, greater than 3000 pounds/square inch, to achieve any kind of sensitivity and tends to be more complicated since this technique requires a sealed diaphragm, which is cost prohibitive and difficult to integrate into a production engine. Fabry-Perot cavities are generally exposed to the combustion chamber and not compatible with the high temperatures in this environment. In addition, they are end-mounted, intensity-based devices and as such require a separate fiber for each cylinder adding cost to the overall system.
Accordingly, a need exists for a simple, cost effective fiber optic measurement device which can withstand the harsh engine environment.